Timber beam bracing

[WAstruc10]

I have a cantilevered PSL 7x20 beam, supported at two locations by PSL 7x7 columns. The problem is that my first support (not at cantilever) has lateral bracing but the support at the beginning of the cantilever has no bottom chord bracing, and cannot due to architectural constraints. Also, hidden connections must be used. My question is: can I rely on a bolted knife plate from column-to-beam to provide lateral-torsional resistance? My engineering instincts say no, but it seems that there are two mechanisms working against a torsional failure: first a tension/compression couple (tension in the knife plate, compression field between the top of column and bottom of beam on one side of the knife plate), and second, if the knife plate is thickened, bending resistance would be provided. But in either case it seems I would need to know how much lateral bending to resist. Thank you for any thoughts.

 

[DaveAtkins]

Yes, if you create some continuity between the bottom of the beam and the top of the column, then the column essentially spans from the foundation to the top of the beam, where it is braced, and so the beam is braced at this location also. The architect won't allow some kind of post cap, saddle like connection?

DaveAtkins

 

[WAstruc10]

No saddle permitted, unfortunately. I agree with your logic on analyzing the column with a longer effective length. But do you have a good reference on how to calculate the amount of flexural continuity required at the column-beam interface? For example, would it make conservative sense to assume 2% (or?) of the axial column load applied laterally at that joint?

 

[DaveAtkins]

Yes, the old 2% rule was reaffirmed by Salmon and Johnson in "Steel Structures, Design and Behavior." I would apply 2% of the axial force to the top of the column, where you are preventing lateral buckling, and design for the moment at the joint in question.

DaveAtkins